GIF is a color restricted format with on/off support for transparency. There's really no settings you can tweak to output a better GIF. You have a drop-down during GIF sequence export to allow transparency, select the amount of colors (up to 256) and you can choose dither to save a little filespace. The only 2 things in there that really matter are dither and amount of colors used to export.

If your content is very similar (e.g. not a lot of complex gradients/shadows/etc) and is limited in color use turning down the amount of colors used and dithering is your only option.

Make sure you're exporting the GIF via "Publish Settings" so you get a single animated GIF and not file->export->movie because this creates a separate sequence of GIF images for each frame.